(1) Field of the Invention
The present invention relates to a projection type image display device, particularly, a projection type image display device with reduced color unevenness using a flat mirror in its projection system.
(2) Description of the Related Art
In projection type image display devices, for example, liquid crystal projectors, color unevenness can occur in a color synthesis process performed at a crossed prism due to differences in reflectance (transmittance) between P-polarized light and S-polarized light. Measures against the problem are disclosed in Japanese Patent Application Laid-Open Nos. 2005-321544 and 2004-133112. Japanese Patent Application Laid-Open No. 2006-292901 discusses the incident angle to an image projection surface in a projection optical system.
In Japanese Patent Application Laid-Open No. 2005-321544, it is pointed out that color unevenness is caused depending on the combination of polarization properties (Fresnel lenses) inherent to transmissive screens and the state of polarization of incident light, and a method for reducing color unevenness is disclosed in which a pseudo polarization-free state is generated by using a phase plate with a phase difference of 5000 nm.
In Japanese Patent Application Laid-Open No. 2004-133112, it is pointed out that, as a cause of color unevenness, there is a reflectance difference of about 17% (i.e. about 22% for S-polarized light less about 5% for P-polarized light) at a wavelength of 550 nm at a half mirror disposed in a 45-degree position in a prompter (observation unit), and use of a wavelength plate for reducing color unevenness is proposed.
Japanese Patent Application Laid-Open No. 2006-292901 discloses, as a super wide-angle projection optical system with a shortened projection distance, a projection optical system of a diagonal projection type in which an image is projected in a diagonal direction with respect to an image projection surface.